CHAPTER XXIII 



Transformation of Sulfur by Microorganisms 



Sources of sulfur in the soil and processes of transformation. In 

 addition to carbon and nitrogen, there are a number of elements which 

 are of prime importance in the growth of plants and microorganisms. 

 We need mention only sulfur, phosphorus, potassium, iron, calcium 

 and magnesium. The transformation of sulfur by microorganisms 

 will be discussed at this point not because this element is more important 

 than the others, but because, next to carbon and nitrogen and except 

 for oxygen and hydrogen, it can be used by certain organisms for 

 energy purposes and is required by the majority of organisms for 

 structural purposes. Certain forms of sulfur may also be used as sources 

 of oxygen under anaerobic conditions. Sulfur is more similar to nitrogen 

 than any other element in the many transformations that it enters and 

 in the types of microorganisms which produce these transformations. 

 One finds in the sulfur cycle apparent duplications of the processes 

 associated with the nitrogen cycle. 



Sulfur occurs in the soil and may be introduced there in the form of 

 organic and inorganic compounds. The latter comprise elementary 

 sulfur, sulfides and sulfates. The organic matter added to the soil 

 contains from 0.1 to 0.5 per cent of sulfur, as shown in table 61. 1 



The sulfur is present in the plant, chiefly in the cystine group of the 

 protein molecule. When large quantities of sulfates are present in 

 the soil, the plant may also contain sulfur in the form of sulfate. Cer- 

 tain plants contain various volatile sulfur compounds, including cer- 

 tain glucosides, such as sinigrin 



/OSO,K 



C 3 H 6 N:< 



X S • C«Hn0 6 



which is decomposed in the soil to mustard oil (C 3 H 5 NCS), glucose 

 and potassium acid sulfate. 2 



1 Hart, E. B., and Peterson, W. H. Sulphur requirements of farm crops in 

 relation to the soil and air supply. Wis. Agr. Exp. Sta. Res. Bui. 14. 1911. 



2 Peterson, W. H. Forms of sulfur in plant materials and their variation 

 with the soil supply. Jour. Amer. Chem. Soc, 36: 1290-1300. 1914. 



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